1. Field of the Invention
The present invention provides a light emitting device material useful as a fluorescent dye and a light emitting device using the same, which can be used for display elements, flat panel displays, backlights, lighting, interiors, signs, signboards, electronic cameras, light signal generators, and the like.
2. Background of the Invention
There has recently been considerable research on organic thin-film light emitting devices which emit light when electrons injected from a cathode and holes injected from an anode recombine within an organic luminous body interposed between the cathode and the anode. This light emitting device has been attracting attention because of such a feature that it is thin and capable of emitting high-luminance light under a low driving voltage and emitting multicolor light through selection of an emissive material.
Since C. W. Tang et al. of Eastman Kodak Company showed that an organic thin-film light emitting device emits light at a high luminance, many research institutes have studied this technique. The typical structure of an organic thin-film light emitting device proposed by a research group of Eastman Kodak Company is such that a hole-transporting diamine compound, an emissive layer made of tris(8-quinolinolato)aluminum(III), and a cathode made of a Mg:Ag alloy are formed sequentially on an ITO glass substrate, and the device is able to emit green light of 1,000 cd/m2 at a driving voltage of about 10 V (see Applied Physics Letters, USA, 1987, Vol. 51, No. 12, pp. 913-915).
Moreover, it has intensively been studied to apply the organic thin-film light emitting device as a display or the like since various luminescent colors can be obtained by using various fluorescent materials in the emissive layer. Of emissive materials of the three primary colors, research on green emissive materials is at the most advanced stage and intensive study is being performed so as to improve characteristics of red emissive materials and blue emissive materials.
One of the greatest objects with organic thin-film light emitting devices is to obtain luminance efficiency and color purity of a device at satisfactory levels simultaneously. With regard to a red light emitting device, there are few red emissive materials capable of providing a device that is high in luminance efficiency and excellent in color purity. For example, a pyrromethene compound, which is a compound that exhibits high luminance light emission, is known as a red dopant material (see Japanese Unexamined Patent Publication No. 2000-208265). It is also known that introduction of an aromatic ring or the like into a pyrromethene skeleton produces red light emission (see Japanese Unexamined Patent Publication No. 2003-12676). Moreover, a technique of the use for a red light emitting device of a thermally stable pyrromethene derivative with which a margin of a decomposition temperature and a sublimation temperature has been secured has been disclosed (see Japanese Unexamined Patent Publication No. 2005-53900). However, none of them achieves both luminance efficiency and color purity sufficiently.